This invention relates to an improvement applicable to a nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing a mixture of PuO2 and UO2 (Hereinafter referred to as a MOX nuclear fuel standing for a mixed oxide nuclear fuel) as the nuclear fuel and light water as the moderator/coolant. More specifically, this invention relates to an improvement brought out for the purpose to reduce the production cost of the MOX nuclear fuel assembly and to increase the value of the spent fuel of the MOX nuclear fuel assembly.
Nuclear reactors available in the prior art are predominantly thermal neutron reactors wherein the nuclear fuel is a plurality of enriched UO02 pellets containing fissionable U235 more than the natural U and the moderator/coolant is light water. In order to prevent chemical reaction from occurring between the UO02 pellets and light water, disc shaped UO2 pellets having an approximate diameter of 1 cm and an approximate height of 1 cm are piled in a tube made of a zircaloy sheath, the zircaloy tube confining plural shaped UO2 pellets being called a nuclear fuel rod. Along the external surface of the nuclear fuel rods, water is allowed to flow for the purpose to act as the moderator/coolant. A light water reactor having a capacity of e.g. 1,100 MW has as many as 50,000 nuclear fuel rods arranged in the reactor core cavity having a diameter of approximately 5 m in parallel one another, remaining space for allowing water to flow between each nuclear fuel rod. Since the quantity of the nuclear fuel rods is so many, it is inconvenient to treat them as a whole. Thus, a concept of grouping some quantity of the nuclear fuel rods, e.g., 60 through 100 nuclear fuel rods, was brought out and the group of nuclear fuel rods was defined as a nuclear fuel assembly. A light water reactor having a capacity of e.g. 1,100 MW has approximately 760 nuclear fuel assemblies. Plural spaces remain between the nuclear fuel rods for allowing water to flow in the nuclear fuel assemblies and for allowing control rods, e.g., boron rods and the like, to be arranged in the nuclear fuel assemblies.
Requirements for the foregoing nuclear fuel assembly are tabulated below.
1. The burn-up of a nuclear fuel assembly is required to be sufficiently large e.g. 40 through 55 GWd/ton during the actual lifetime of 4 through 5 years.
2. The distribution of volumetric power density or actually the heat distribution in the reactor core is required to be uniform for the entire space of each nuclear fuel assembly and resultantly for the entire space of the reactor core.
3. Each nuclear fuel assembly is required to be durable to safely confine the fission products within the nuclear fuel assembly.
Referring to a drawing, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employed for a thermal reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant, will be described below.
Referring to FIG. 1 illustrating a horizontal cross-section of a nuclear fuel assembly designed to be employed for a UO2/light water reactor available in the prior art, symbols 1, 2, 3, 4 and 5 show 5 independent kinds of UO2 nuclear fuel rods. Each kind of UO2 nuclear fuel rods contains UO2 nuclear fuel of which the enrichment grade is same to each other or the UO2 nuclear fuel contained in a kind of UO2 nuclear fuel rods has a single enrichment grade, although the enrichment grade is different for each kind. The enrichment grade is reduced from symbol 1 toward symbol 5. The quantities of symbols 1, 2, 3, 4 and 5 are 10, 30, 4, 24, and 4 respectively. Each of the UO2 nuclear fuel rods is approximately 4 m in length and approximately 11 mm in the external diameter. The nuclear fuel assembly has a cross-section which is a square of which the length of each side is approximately 15 cm. Symbol W shows a water rod having sides of approximately 4 cm. As is shown in the drawing, UO2 nuclear fuel rods having a higher enrichment grade are arranged generally at the center of the reactor core and those having less enrichment grades are arranged surrounding the symbol 1 to turn out the enrichment grade gradually less in the outer space.
The philosophy of the arrangement of nuclear fuel rods in the nuclear fuel assembly designed for a UO2/light water reactor is summarized below.
1. Many kinds of UO2 nuclear fuel rods, each kind being different in enrichment grade, are employed.
2. The enrichment grade of the UO2 nuclear fuel contained in each kind of the UO2 nuclear fuel rods is same and the enrichment grade is selected to meet the requirements to make the distribution of the volumetric power density or actually the heat distribution in the reactor, uniform for the entire space of each nuclear fuel assembly, and resultantly for the entire space of the reactor core. Specifically, the enrichment grade of each nuclear fuel rod is selected to be different from one another and is made higher at the center of the reactor core to be made gradually less at the outer space.
It is well known that U235 contained in the UO2 nuclear fuel contributes for generation of heat by fission and U238 contained in the UO2 nuclear fuel absorbs neutrons and transits itself to isotopes of a higher order or having a higher atomic number e.g. Pu239, Pu240, Pu241 or Pu242.
Since Pu239 and Pu241 alone are fissionable, they are employable for causing fission either in the reactor currently employed or in other reactors after Pu239 and Pu241 are recovered from the spent fuel thereof by a spent fuel reprocessing process.
A mixture of oxides of the foregoing isotopes of a higher order than U235 or having a higher atomic number than U e.g. Pu239, Pu240, Pu241 and Pu242 and of oxides of U is called a MOX nuclear fuel standing for a mixed oxide nuclear fuel.
Following the foregoing background, brought out was a concept of a nuclear reactor which is allowed to employ either an oxide of UO2 alone or a MOX nuclear fuel. A MOX nuclear fuel assembly produced from the MOX nuclear fuel is the object of this invention.
Referring to a drawing, an exemplary arrangement of nuclear fuel rods in a MOX nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 alone or MOX nuclear fuel will be described below.
Referring to FIG. 2 illustrating a horizontal cross-section of a nuclear fuel assembly designed to be employable either for a UO2/light water reactor available in the prior art or for a MOX nuclear fuel/light water reactor available in the prior art, symbols P1, P2, P3 and P4 show 4 independent kinds of MOX nuclear fuel rods. Each kind of MOX nuclear fuel rods contains fissionable Pu having a single enrichment grade or the same enrichment grade. The fissionable Pu enrichment grade differs from one to another for each kind of nuclear fuel rods and is reduced from symbol P1 wherein the fissionable Pu enrichment grade is approximately 5% toward symbol P4 wherein the fissionable Pu enrichment grade is much less. The quantity of MOX nuclear fuel rods represented by symbols P1, P2, P3 and P4 is 20, 17, 8 and 3 respectively. Each of the MOX nuclear fuel rods is approximately 4 m in length and approximately 12 mm in the external diameter. The nuclear fuel assembly has a cross-section which is square of which the length of each side is approximately 15 cm. Symbols G1 and G2 show gadolinium fuel rods. The quantity of the gadolinium fuel rods G1 and G2 is 4 and 8 respectively. Thus, the MOX nuclear fuel rods accounts for 80% of the total quantity of the nuclear fuel rods employed in the nuclear fuel assembly. The function of the gadolinium fuel rods is to restrict fission from occurring at the beginning of the reactor operation period. In other words, the gadolinium fuel rods are effective to reduce the possibility of occurring fission at the beginning of the reactor operation period but thereafter they lose such a function and transit themselves to fissionable fuels. In this sense, gadolinium fuel rods are included in the category of the nuclear fuel. Symbol W shows a water rod having sides of approximately 3 cm. As is shown in the drawing, the MOX fuel rods having a higher enrichment grade of the fissionable PU-s are arranged at the center of the reactor core and those having less enrichment grades of the fissionable PU-s are arranged surrounding the symbol P1 to turn out the enrichment grade of the fissionable PU-s gradually less in the outer space. The average magnitude of the enrichment grade of the fissionable PU-s is approximately 3%. It is needless to emphasize that selection of UO2 fuel alone, MOX nuclear fuel alone or a combination of UO2 fuel and MOX nuclear fuel is allowed for a reactor which is designed to burn either UO2 alone or MOX nuclear fuel.
As is clear from the foregoing description, the arrangement of the MOX nuclear fuel rods in the foregoing nuclear fuel assembly employable for a thermal reactor which is designed to allow burning either UO2 alone or MOX nuclear fuel available in the prior art is a simple replacement of the UO2 fuel rods with the MOX nuclear fuel rods applied to the foregoing nuclear fuel assembly employable for a thermal reactor which is designed to allow burning UO2 alone or MOX nuclear fuel available in the prior art, without paying particular attention to the difference in nature of the UO2 fuel and the MOX nuclear fuel. Needless to emphasize that
As a result, the philosophy of the arrangement of the nuclear fuel rods in the nuclear fuel assembly employable either for a reactor designed to burn UO2 fuel or for a reactor designed to burn MOX nuclear fuel available in the prior art can be summarized below.
1. Employed are many kinds of MOX nuclear fuel rods each of which kinds is different from one another in grade of fissionable Pu-s enrichment.
2. The magnitude of the fissionable Pu-s enrichment grade of the many kinds of the MOX nuclear fuel rods varies 6% or less. In other words, the enrichment grade is relatively less.
3. The ratio of the total quantity of the MOX nuclear fuel rods contained in an assembly and the entire quantity of the nuclear fuel rods contained in an assembly is as high as 80%.
4. Each kind of the MOX nuclear fuel rods is selected to meet the requirements to make the distribution of volumetric power density or actually the heat distribution in the reactor uniform for the entire space of each nuclear fuel assembly. Specifically, the enrichment grade of each MOX nuclear fuel rod is selected to be different from one another and is made higher at the center of the reactor core to be made gradually less at the outer space.
This philosophy is resultantly involved with the following drawbacks, predominantly caused by the difference in nature of UO2 fuel and the MOX nuclear fuel, described below.
1. In the case of U, since the raw material of the UO2 fuel or the natural U contains quite little quantity or 0.7% of the fissionable U235, a process for obtaining U containing an appropriate quantity of fissionable U235 (U having a larger enrichment grade than that of the raw material) or a process for enriching U requires a considerable amount of cost. Conversely, in the case of the MOX nuclear fuel, since the raw material or Pu-s recovered from the spent fuel by conducting a spent fuel reprocessing process contains a large quantity or 60 through 70% of the fissionable Pu239 and Pu241, a process for obtaining MOX nuclear fuel containing an appropriate quantity of Pu239 and Pu241 (MOX nuclear fuel having a smaller enrichment grade than that of the raw material) or a process for reducing the enrichment grade requires a considerable amount of cost. In other words, although in the case of UO2 fuel, a less magnitude of enrichment grade is beneficial, in the case of MOX nuclear fuel, a large magnitude of enrichment grade is beneficial. Thus, the foregoing fuel assembly available in the prior art is involved with a drawback wherein the production cost thereof is considerably large.
2. Production of plural kinds requires a cost larger than that for production of a single kind. Thus, the nuclear fuel assembly having many kinds of MOX nuclear fuel rods each of which kinds has plural fissionable Pu-s enrichment grades different from one another, is disadvantageous from the economical viewpoints.
3. Although the spent fuel of UO2 fuel rods is valuable, the spent fuel of the MOX fuel rods has a lesser value for use the material as a nuclear fuel, because absorption of neutron by UO2 produces Pu239, Pu240, Pu241 and Pu242 including the fissionable Pu239 and Pu241, absorpotion of neutron by the MOX nuclear fuel produces only Pu-s having a higher order most of which are non-fissionable Pu240 and Pu242. Since the foregoing nuclear fuel assembly available in the prior art uses many MOX nuclear fuel rods, a resulting drawback is that much of the spent fuel has a lesser value.
The object of this invention is to remove the foregoing drawbacks and to provide an improvement applicable to a nuclear fuel assembly employable either for a thermal neutron reactor designed to employ UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor designed to employ the MOX nuclear fuel as the nuclear fuel and light water as the moderator/coolant, wherein the production cost thereof is less expensive and the spent fuel thereof is more valuable than the spent fuel of a nuclear fuel assembly employable for a reactor designed burning either UO2 fuel or the MOX fuel available in the prior art.
For the purpose to achieve the foregoing objects, a nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing the MOX nuclear fuel as the nuclear fuel and light water as the moderator/coolant in accordance with this invention is designed, based on the philosophy entirely different from that of the prior art.
The philosophy specific for the nuclear fuel assembly of this invention will be summarized below.
1. Only one kind of MOX nuclear fuel rods or MOX nuclear fuel rods having only one magnitude of enrichment grade of the fissionable Pu-s is employed.
2. The enrichment grade of the fissionable Pu-s contained in the only one kind of the MOX fuel rods is selected to be much higher than that of the fissionable Pu-s contained in the MOX fuel rods of the nuclear fuel assemblies available in the prior art.
3. The total quantity of the MOX nuclear fuel rods is selected to be much less than that of the prior art.
4. The burn-up obtained by the total quantity of the MOX nuclear fuel rods is identical to or larger than that of the prior art.
More specifically, the magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241 of the only one kind of the MOX nuclear fuel rods varies 5 through 15 weight %, more preferably 10 through 15 weight %, with respect to that of the entire volume of Pu-s including the fissionable and non-fissionable Pu-s, and the quantity of the MOX nuclear fuel rods with respect to the total quantity of the entire fuel rods, varies 20 through 40%, more preferably 20 through 25%.
The function and advantages of this invention will be described below.
Firstly, as was described earlier, the raw material of the MOX nuclear fuel is Pu-s recovered from the spent fuel of UO2 fuel which was burnt in a reactor, and the recovered Pu-s contain 60 through 70% of the fissionable Pu-s or Pu239 and Pu241. Since the MOX nuclear fuel having such a high enrichment grade can not be acceptable by a thermal reactor, reduction of the enrichment grade is essential for production of a MOX nuclear fuel rod. Thus, the cost for producing a MOX nuclear fuel rod containing a less quantity of the fissionable Pu-s or having a less magnitude of enrichment grade is more expensive than the cost for producing a MOX nuclear fuel rod containing a larger quantity of the fissionable Pu-s or having a larger magnitude of enrichment grade. Thus, the less the enrichment grade is, the more the production cost is, for a MOX nuclear fuel. In contrast, the raw material of the UO2 fuel is the natural U containing quite small quantity or 0.7% of the fissionable U235. Thus, the more the enrichment grade is, the more the production cost is, for a UO2 fuel. For the purpose to reduce the production cost of a MOX nuclear fuel rod, MOX fuel having a fairly higher enrichment grade are employed for the nuclear fuel assembly of this invention. As a result, the production cost of the nuclear fuel assembly of this invention is less expensive than the prior art.
Secondly, it is quite natural that the production cost of nuclear fuel assemblies employable for a reactor is inclined to increase following an increase in the quantity of kinds of the nuclear fuel rods. In other words, mass production of only one kind of a product is more economical than production of many different kinds of products. Since the kind of the MOX fuel rods employed in the nuclear fuel assembly of this invention is only one, it is clear that the production cost of the nuclear fuel assembly of this invention is less expensive than the prior art.
Thirdly, since the total quantity per hour of neutrons required to be emitted in a reactor is predetermined for the specific reactor, an increase in the enrichment grade of the MOX nuclear fuel results in a reduction in the quantity of the MOX nuclear fuel rods. By this reason, the quantity of the MOX nuclear fuel rods of this invention is reduced. This resultantly causes an advantage to reduce the quantity of the spent fuel of which the value is much less. An increase in the enrichment grade of a small quantity of the MOX rods is accompanied by a possibility of causing a local peaking problem. Thus, the magnitude of the enrichment grade of the MOX rods employed in the nuclear fuel assembly of this invention is determined, paying a serious attention not to cause the local peaking problem.
Fourthly, referring to a drawing, the reason why the value of the spent fuel of this invention is larger than that of the spent fuel of the prior art, will be described below.
The reason why the value of the spent fuel of the MOX nuclear fuel is less for use the material as a nuclear fuel will be discussed below.
FIG. 3 illustrates the cross-section of U235 and Pu239, Pu240, Pu241 and Pu242. The cross-section of the fission of U235 is large in the energy range corresponding to the thermal neutron range (low energy range) and small in the higher energy range. In contrast, the neutron absorption cross-section of Pu239, Pu240, Pu241 and Pu242 is large in the epi-thermal neutron range (high energy range), due to the resonance absorption peak recognized in the high energy range. This means that although a considerable quantity of neutrons encountering U235 have much possibility to cause fission, most quantity of neutrons encountering Pu239, Pu240, Pu241 and Pu242 are absorbed by them to simply increase the order of the Pu-s.
This phenomenon can be accounted for by Table 1 comparing Pu composition contained in the MOX fuel assembly (Pu composition of the total quantity of Pu-s contained in a MOX fuel assembly) before being burnt in a reactor and Pu composition contained in the MOX fuel assembly (Pu composition of the total quantity of Pu-s contained in a MOX fuel assembly) after being burnt in a reactor.
The table shows the reduction of the sum of Pu239 and Pu241(reduction from 67% to 47%) is considerably large. This means the comparative quantity of sum of Pu239 and Pu241 with respect to the entire quantity of Pu isotopes contained in the spent fuel of the MOX fuel assembly is small.
On the other hand, insofar as the comparative quantity of sum of Pu239 and Pu241 with respect to the entire quantity of Pu isotopes contained in each fuel rod is concerned, the situation is different. Namely, Table 2 (Pu composition contained in a UO2 fuel rod after being burnt and a MOX fuel rod after being burnt in a reactor) shows that although the comparative quantity of sum of Pu239 and Pu241 with respect to the entire quantity of Pu isotopes contained in the spent fuel of a UO2 fuel rod is 68%, the comparative quantity of sum of Pu239 and Pu241 with respect to the entire quantity of Pu isotopes contained in the spent fuel of a MOX fuel rod is 39%.
This means that however a MOX fuel rod reduces its value by being burnt in a reactor, a UO2 fuel keeps its value even after it was burnt in a reactor by the following reasons.
1. A MOX fuel rod is required to contain a specific quantity of fissionable Pu isotopes. If the comparative quantity of sum of Pu239 and Pu241 with respect to the entire quantity of Pu isotopes contained in a MOX fuel rod is less, it needs a larger quantity of fissionable and non-fissionable Pu isotopes. Since non-fissionable Pu isotopes emit xcex1 ray as well, a larger magnitude of radioactive confinement will be required for establishments of spent fuel reprocessing units which will be employed later.
2. Since Pu240 has a large cross-section for absorption of neutrons, an increase in the quantity of Pu240 requires an increase in the quantity of fissionable Pu isotopes.
3. Since Pu240 produces Pu241 which is fissionable, this phenomenon resultantly increases the future potential magnitude of fission. This resultantly causes a complicated problem for designing a reactor in long term viewpoints.
Since the MOX fuel assembly of this invention has a rather small quantity of MOX fuel rods containing a fairly large quantity of fissionable Pu-s, the MOX fuel assembly resultantly has a large quantity of UO2 fuel rods. Thus, the spent fuel of the MOX fuel assembly of this invention contains large volume of fissionable Pu-s. In this sense, the spent fuel of the MOX fuel assembly of this invention is valuable from the viewpoints of using the material as the fuel of a nuclear reactor.